We have developed a stopped-flow flow cytometer with subsecond resolution to be used in kinetic analysis of cell activation. This instrument is extending the application of flow cytometry to important biological molecules that interact with cell surface receptors. The instrument is expected to make it possible to examine the interaction of hormones and other small molecules with receptors when the affinities of the binding are too low to be examined by other methods. We have modified a commercial stepper-motor-driven, stopped-flow mixing system, and integrated the mixer with a flow cytometer. This device is being used to detect ligand-receptor interactions in living cells and is expected to extend the use of flow cytometers to new communities of biomedical and health scientists. In the previous year we improved the system by decreasing the dead volume between the exit valve of the delivery system and the port of the flow system so as to achieve delivery times under 300 msces. A description of the device has been published (Nolan et al, Cytometry 1995) We have recently modified the mixing device to permit two sequential mixing operations and added another phototube tto the cytometer which will allow analysis of the complex time-dependent biological processes that occur during the activation of living cells. Measurements are being made on cells expressing ligands of diffrerent afifinities and mutant receptors.